The invention relates to a light grid for detecting objects within a monitored zone. Such a light grid is know to include a predetermined number of transmitters for emitting light rays, as well as receivers, wherein a beam axis if formed with respectively one transmitter and a receiver assigned to this transmitter. An evaluation unit generates an object detection signal in dependence on the signals received and present at the receiver outputs.
A light grid of this type is disclosed in German Patent document DE 39 39 191 A1. The light grid described therein is provided with a transmitter in the form of infrared transmitting diodes arranged in a row, which are activated one after another periodically and cyclically and emit pulse-pause modified pulses with an identifier. The identifier of a selected infrared transmitting diode differs from the identifier of the remaining infrared transmitting diodes. The receiver is formed with a corresponding row of periodically and cyclically activated photodiodes, clocked by a free running oscillator, wherein each photo diode is assigned to one of the infrared transmitting diodes. A signal “protective zone not free” is generated in an evaluation unit if at least one light beam from the transmitter to the receiver is interrupted. The evaluation unit furthermore synchronizes the cyclical activation of the photodiodes in the receiver with that of the transmitter, based on the identifier received by the selected infrared transmitting diode.
The transmitters for this light grid are activated cyclically, one after another, via a feedback shift register. To ensure a failsafe operation, the correct operation of this shift register is checked with the aid of an integrated error-detection circuit.
The receivers are also cyclically activated with the aid of shift registers, wherein the function of the shift register is checked in the evaluation unit with a two-channel design.
The disadvantage of this light grid is that it can only be synchronized to the selected transmitter, which transmits an identifier that differs from the identifier imprinted on the light rays transmitted by all other transmitters. With a light grid having a muting and blanking function, predetermined beam axes that are not used for the object detection are typically blanked out. These beam axes can or even must be continuously interrupted. For that reason, the beam axis of the selected transmitter for the light grid according to German patent document DE 39 39 191 A1 generally cannot be used to form blanking or muting zones since it can otherwise no longer be used for synchronizing the light grid, meaning a light grid operation would no longer be possible.
A further disadvantage of this light grid requires an undesirably complex circuitry for ensuring a failsafe operation, meaning for the error monitoring necessary when using the light grid in the area of safety technology.